Compared with other surgical methods, a primary advantage of ultrasound surgery is its noninvasive nature. Ultrasound allows diagnostic and therapeutic procedures to be accomplished either wholly from means external to the body, or with minimal dependence on procedures no more invasive than current laproscopic techniques. Being noninvasive, the cost advantages, both in hospital stay and in surgical preparation time, are readily apparent. In addition, the lack of cosmetic disfigurement and risk of infection are both significant advantages for ultrasound procedures.
Ultrasound can be utilized for diagnostic imaging, wherein an ultrasound transducer is used to generate ultrasonic waves which are directed at a region of interest in a patient. The transducer then receives reflected ultrasonic waves from the region and converts the received waves into electrical signals from which an image is generated. Ultrasound has also been used in various therapeutic applications. One such application, thermally-based ultrasound surgery, involves applying ultrasonic waves to a targeted treatment volume, such as a tumor, in order to heat the treatment volume and create a lesion. An example of such an application can be found in U.S. Pat. No. 5,694,936 issued to Fujimoto et al. Another application of therapeutic ultrasound is in the treatment of vascular thrombosis as seen, for example, in U.S. Pat. No. 5,648,098 issued to Porter. Unfortunately, the otherwise beneficial results of both diagnostic and therapeutic ultrasound procedures are often made unpredictable by the phenomenon of acoustic cavitation.
Acoustic cavitation is a term used to define the interaction of an acoustic field, such as an ultrasound field, with bodies containing gas and/or vapor. This term is used in reference to the production of small gas bubbles, or microbubbles, in the liquid. Specifically, when an acoustic field is propagated into a fluid, the stress induced by the negative pressure produced can cause the liquid to rupture, forming a void in the fluid which will contain vapor and/or gas. Acoustic cavitation also refers to the oscillation and/or collapse of microbubbles in response to the applied stress of the acoustic field.
The induced oscillation of microbubbles can generally be categorized as noninertial cavitation or as inertial cavitation. Noninertial cavitation appears at very low acoustic pressure amplitudes, as soon as microbubbles are present in a tissue. In noninertial cavitation, the walls of the microbubbles oscillate at the frequency of the ultrasound field generally without damaging surrounding cells, but considerably disturbing ultrasound transmission by reflecting or scattering incident waves. Inertial cavitation appears rather suddenly at higher incident pressures, thus defining a cavitation onset threshold. In inertial cavitation, microbubbles expand to reach a critical size after which the collapse is driven by the inertia of the surrounding fluid, thus the term "inertial" cavitation. Microbubble size is a determining factor in the degree of response to the ultrasound field, such that microbubbles are highly resonant oscillators at certain drive frequencies. Microbubble oscillation can be sufficiently violent to produce mechanical or thermal damage on surrounding tissue, thereby creating lesions.
In current practice, significant steps are usually taken to avoid cavitation, as described in U.S. Pat. No. 5,573,497 issued to Chapelon. Typically, cavitation is only permitted where it can be very carefully controlled and localized, such as at the end of a small probe or catheter as in U.S. Pat. No. 5,474,531 issued to Carter. The primary reason for avoiding cavitation is that thresholds for inducing cavitation of microbubbles are unpredictable due to the diversity of microbubble sizes and quantities in different tissues. Uncontrolled cavitation hinders the penetration of ultrasonic waves into tissue, and can lead to uncontrolled tissue destruction outside the intended treatment volume. As a result, surgical protocols have been formulated which attempt to increase cavitation onset thresholds in most diagnostic and therapeutic applications.
Cavitation occurs more easily at low frequencies of ultrasound transmission, with the cavitation threshold increasing as the frequency of ultrasonic waves is increased. Therefore, the predominant method of controlling cavitation during ultrasound procedures has been to utilize high frequency ultrasonic waves, as disclosed, for example, in U.S. Pat. No. 5,601,526 issued to Chapelon et al. and in U.S. Pat. No. 5,558,092 issued to Unger et al. However, this approach is not without drawbacks, as high frequency ultrasound cannot penetrate as far in soft tissue or through bone. In addition, high frequency ultrasound often has the detrimental effect of excessively heating tissues located between the ultrasound transducer and the intended treatment volume.